Integrated optical guides of glass are interesting for fabricating different types of integrated optical components, since they have reduced attenuation, are compatible with commercially available optical fibers and can be produced at relatively low cost.
The most widely used technique for fabricating such guides is ion exchange, thanks to its simplicity and to the flexibility allowed in the choice of size and geometric characteristics of the resulting guide. Exchange takes place between Na.sup.+ ions, always present in ordinary glasses such as soda-lime and boro-silicate glasses, and other monovalent ions such as Li.sup.+, Cs.sup.+, Rb.sup.+, Ti.sup.+ and more usually K.sup.+ and Ag.sup.+ : owing to the smaller size (e.g. of Li.sup.+ ions) or the higher electronic polarizability (all others of the above mentioned ions) as compared to Na.sup.+ ions, a refractive index increase is obtained in the regions involved in the exchange and these regions can thus exert a guiding action on the light radiation launched into them.
Generally ion exchange is performed by using molten salts of the concerned elements, as described in the papers "Glass waveguides by ion exchange: a review" by T. Findakly, Optical Engineering, Vol. 24, No. 2, March-April 1985, pages 244 to 250 and "Ion-Exchanged Glass Waveguides. A Review", by R. V. Ramaswamy and R. Srivastava, Journal of Lightwave Technology, Vol. 6, No. 6, June 1988, pages 984-1001. These methods suffer from some disadvantages. First, they require a series of complex operations prior to contacting the substrate with the molten salts, since masking of the area where the exchange is not to occur is necessary. This operation is rather long, chiefly if the guide geometry is complex. Secondly, all process parameters (glass composition, nature and concentration of the exchanging ions, temperature, duration, material purity, external electric field etc.) have to be carefully controlled in order to obtain guides with the desired characteristics, since the tefractive index depends in a complex way on these parameters. Moreover, refractive index variation at the guiding zone boundaries is always rather gradual (even in case of a step refractive index profile) and involves a band whose width cannot be reduced under a certain value.